1. Field of the Invention
This invention relates generally to a process for the treatment of wastewater sludges and biosolids in order to achieve the highest classification (Class A) for sludge and sludge products with respect to pathogen removal and vector control and in particular, to an improved process for wastewater treatment that utilizes ambient and internal sludge pressure variation such as vacuum and high pressure extrusion in conjunction with pH conditioning to achieve U.S.E.P.A. Class A sludge quality in conjunction with reducing the effects of putrescible organic compounds in achieving vector attraction reduction.
2. Description of the Prior Art
Pathogens are microbial organisms that cause disease and enter the environment via raw and poorly treated sewage discharges, in wastewater sludge, and from leachates and runoff. Bacteria and viruses are the main health concerns. Protozoans that cause dysentery, parasitic flatworms that cause trichinosis and schistosomiasis, nematodes and cestodes that cause roundworm, threadworm and tapeworm manifestations are also a problem.
Wastewater treatment resulting typically from sewage has a principal bacteria, the Fescherichia coli, the fecal coliform found in all human beings; salmonella, responsible for typhoid and paratyphoid fevers, food poisoning, and gastroenteritis; shigella, bacteria dysentery; Clostridium food poisoning, Saphylococcus aureus, wound infections and food poisoning; mycobacterium, tuberculosis; and leptospira, Weil's disease and jaundice. Sewage treatment has become mandatory throughout the United States in most areas. This can result in primary treatment, secondary treatment, and tertiary treatment. The sewage treatment separates out solids from water, leaving wastewater sludge. Heavy metals, organics, oil, grease, bacteria, viruses, and protozoans all accumulate in sludge. If the wastewater sludge is to be "reused" in a beneficial manner, it must be conditioned, dewatered, digested, and have pathogens significantly reduced. If it is to be used as fertilizer, it must be treated to destroy common pathogens. The sludge should also be treated to remove odors and reduce or eliminate vector attraction.
One of the most important pathogen removal standards to be met must comply with 40 CFR Part 503, Sewage Sludge Disposal Standards. Class A standards require the reduction of the viable helminth ova (parasitic worm eggs) to below detectable levels. Other pathogens that must be removed to pass the Class A standards are the various types of bacteria and viruses discussed above. Treatment of pathogens such as bacteria and viruses and their elimination are not necessarily achieved with the same environmental agents used to reduce and destroy viable helminth ova. Bacteria and viruses can normally be effectively destroyed by boosting the sludge to a Ph of greater than 12 using alkaline agents such as lime, cement, cement kiln dust (CKD), some fly ashes or similar alkaline or caustic agents. However, helminth ova require different treatment which includes various combinations of higher temperatures and time in order to kill them. The high pH used to control bacteria and viruses may also provide a synergistic benefit in removing helminth ova by reducing the temperature and time required for the destruction of the helminth ova.
Because of the current high volume of sludge resulting from sewage treatment throughout the United States, reconditioning of the sludge is extremely important and must be cost effective. Ideally, achieving a Class A standard for sludge at low cost should greatly solve the problem of sludge removal and reuse.
The highest pathogen removal standards are currently recognized as "Processes to Further Reduce Pathogens" (PFRP) standards and will be equivalently known as the Class A sludges or Class A sludge products. The standards proposed are either a direct measurement of pathogens to levels equal to or less than three salmonella, one plaque-forming virus unit, one protozoan organism, and one helminth ova per gram of volatile suspended solids. A different approach that is less complicated can eliminate the need for direct measuring of pathogens for Class A can be met by raising the temperature of the sludge to 53.degree. C. for five days, 55.degree. C. for three days, or 70.degree. C. for half an hour. The densities of more easily measured indicator organisms must be reduced to levels equal or less than 100 counts per gram of volatile suspended solids for both fecal coliform and fecal streptococci. Treatment technologies that may comply with Class A requirements include composting, thermophilic anaerobic digestion, drying, and heat treatment. Alkaline stabilization methods may also be applicable under the indicator approach if adequate temperatures and contact times can be achieved.
The prior art discloses methods for treating wastewater sludge to achieve E.P.A. standards for PFRP. U.S. Pat. No. 4,781,842, issued to Nicholson on Nov. 1, 1988, describes a method of decontaminating wastewater sludge by raising the pH of the mixture to 12 and above for at least two hours and drying the resulting mixture by an aeration process for one or more days. U.S. Pat. No. 5,013,458, issued to Christy on May 7, 1991, shows a combination of using chemical additives, pH elevation, and pasteurization heat (70.degree. C. for at least 30 minutes for pathogen reduction in waste). U.S. Pat. No. 4,554,002, issued to Nicholson on Nov. 19, 1985, shows a method of beneficiating waste sludge using kiln dust ambient temperature exposure without compacting to produce a disintegratable product which can be applied to land soil. Another U.S. Pat. No. 4,902,431, issued to Nicholson et al. on Feb. 20, 1990, relates to a method of decontaminating wastewater sludge to a level to exceed U.S.E.P.A. PFRP standards using lime or other alkaline materials mixed with the sludge in sufficient quantity to raise the pH of the mixture to 12 and above for a predetermined time and then drying the resultant mixture. Other methods for treating and sterilizing sludge are shown in U.S. Pat. No. 4,270,279, issued to Roediger on Jun. 2, 1981, and to Wurtz in U.S. Pat. No. 4,997,572, issued Mar. 5, 1991.
The present invention provides for a method to treat wastewater sludge cost effectively to achieve the highest pathogen removal standards as recognized in the PFRP and meeting the criteria for reducing the effects of putrescible organic compounds to reduce and control vectors, such as rodents, flies and mosquitos (vector attraction reduction).